Paramyxoviruses are the most important cause of respiratory disease in children. The long-term objectives of our research are to develop means to prevent respiratory disease caused by parainfluenza viruses (PIV). To reach that objective, the proposed research will focus on establishing the three- dimensional structure (3D) of the paramyxovirus HN protein using x-ray crystallography. The role of this protein in virus attachment, penetration, virus spread, and as a major neutralizing antigen makes the structure important for vaccine and drug development. Using a reverse genetics, this proposal also addresses the determinants of PIV host range and tissue tropism. Using genetic and biochemical approaches aims 3 explores the unknown mechanism of PIV budding, a potential target for drug intervention. The specific aims of this proposal are 1) Proposal: Establish a high-resolution 3D structure of HN. 2) Question: What are the virus specific determinants of parainfluenza host range and tissue tropism? 3) Proposal: determine the mechanism of virus budding induced by paramyxovirus membrane proteins. The studies proposed in these aims, HN structure, virus release (budding), and virus spread (host range determinants) are potential intervention targets in the infectious cycle.